Magnetostrictive actuator with center bias

ABSTRACT

Exemplary practice of the present invention provides a magnetostrictive actuator characterized by linear force output and uniform magnetic biasing. A center bias magnet combined with a flux transfer tube produces a uniform magnetic bias down the length of a magnetostrictive component. Depending on the inventive embodiment, the magnetostrictive component may include one magnetostrictive element or a pair of collinear magnetostrictive elements. A center bias magnet, in combination with a flux transfer tube, drives magnetic flux through the magnetostrictive component (e.g., a series of magnetostrictive rods) in opposite directions, while surrounding drive coils apply flux in the same direction through the magnetostrictive component. The net response is substantially linear with respect to the drive coil current. The flux transfer tube applies distributed magnetic flux to the magnetostrictive component at a rate that ensures uniform magnetic flux density down the length of the magnetostrictive component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. nonprovisional patentapplication Ser. No. 15/717,658, filing date 27 Sep. 2017, herebyincorporated herein by reference, entitled “Magnetostrictive ActuatorWith Center Bias,” inventor John E. Miesner.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention relates to magnetostrictive actuators, moreparticularly to magnetostrictive actuators that seek to produce a linearforce output and/or a uniform magnetic bias flux in the magnetostrictiveelements.

Magnetostrictive actuators offer great promise for applications thatrequire a high force output over a wide bandwidth. However,magnetostrictive materials have two characteristics that limit theiruse. The first limiting characteristic is the inherent nonlinearmaterial response in strain to magnetic flux density. Many applicationsrequire a linear force output that has not been achieved by the currentart for magnetostrictive actuators. The second limiting characteristicis the relatively low permeability of magnetostrictive materials insofaras it makes it difficult to achieve a uniform magnet bias down thelength of the magnetostrictive element due to flux leakage.

U.S. Pat. No. 5,451,821 to Teter et al., incorporated herein byreference, teaches a method of compensating for magnetic flux leakageusing magnets outside the drive coils to apply a magnetic fieldperpendicular to the desired bias direction. Teter et al.'s method hasproven to be effective and has been widely adopted in the design ofmagnetostrictive actuators. However, the perpendicular magnets requiredby Teter et al. are relatively large, thus increasing the size and costof an actuator using this method. The perpendicular magnets also causelarge magnetic fields external to the actuator, which are not acceptablein many applications. These external fields cannot be effectivelyshielded by the usual method of surrounding the entire actuator with aferromagnetic case, because doing so would short out the perpendicularmagnet flux.

U.S. Pat. No. 6,891,286 to Flanagan et al., incorporated herein byreference, teaches large axially polarized disk magnets at each end of amagnetostrictive rod to achieve a uniform magnetic flux down the lengthof the rod. However, this approach of Flanagan et al. does nothing toaddress the inherent nonlinear material response, requires largemagnets, and has large external magnetic fields.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a magnetostrictive actuator having linear force output anduniform magnetic biasing in the magnetostrictive elements.

An exemplary embodiment of the present invention is a magnetostrictiveactuator that produces a linear force output and which uses a centerbias magnet and flux transfer tube to produce a uniform magnetic biasdown the length of the magnetostrictive elements without externalmagnetic fields.

An exemplary magnetostrictive actuator according to the presentinvention comprises a coil component, a magnetostrictive componentsituated inside the coil component, an output shaft situated inside themagnetostrictive component, a support member situated inside the coilcomponent and attached to the magnetostrictive component and to theoutput shaft at one longitudinal end of the output shaft, a magnet ringinterposed between the first coil element and the second coil element,and a flux transfer tube contiguous the magnet ring and situated outsidethe magnetostrictive component and inside the coil component and themagnet ring. The coil component, the magnetostrictive component, theoutput shaft, the support member, the magnet ring, and the output shaftare each at least substantially cylindrical and are characterized by thesame geometric longitudinal axis. The coil component is capable ofcarrying current and of producing a fluctuating magnetic field, in themagnetostrictive component, that is proportional to the current carriedby the coil component. The magnetostrictive component magnetostrictivelychanges in shape in accordance with the fluctuating magnetic field. Thesupport member and hence the output shaft move in an axial direction inaccordance with the magnetostrictive changing in shape of themagnetostrictive component. The magnet ring and the flux transfer tube,in combination, are configured to transfer magnetic flux from the magnetring to the magnetostrictive component so as to provide an at leastsubstantially uniform magnetic bias along the longitudinal axis.

According to some inventive embodiments a surrounding ferromagnetic casecompletes the flux path. The flux transfer tube applies distributedmagnetic flux to the magnetostrictive component at a rate that ensuresuniform flux density down the length of the magnetostrictive component.The closed flux path and ferromagnetic case ensure that the externalmagnetic field is insignificant.

U.S. Pat. No. 5,587,615 to Murray et al., incorporated herein byreference, teaches a method to linearize the output of a magneticactuator with force generated across air gaps. Murray arranges two airgaps with the total actuator force equal to the difference of the forcesacross them, and then establishes (i) magnetic bias flux in oppositedirections in the two air gaps and (ii) coil flux in the same directionin the two air gaps. Therefore, as coil flux increases it tends tocancel the bias flux in one gap and add to the bias flux in the othergap.

The inherent force generated across an air gap is quadratic with respectto the total flux across the gap. If the bias flux is Φ_(bias) and thecoil flux is Φ_(coil), then the force in one gap can be written asF=k(Φ_(bias)±Φ_(coil))² where k is a proportionality constant dependenton the geometry. The net force in the two gaps can be written asF_(net)=k[(Φ_(bias)+Φ_(coil))²−(Φ_(bias)−Φ_(coil))²]. Simplifying thisequation yields F_(net)=4kΦ_(bias)Φ_(coil). Thus, the net output forceis linear with respect to the coil flux.

The response of a magnetostrictive material such as Terfenol issubstantially quadratic with respect to magnetic flux density throughthe material up to the flux level at which it begins to saturate. Thepresent invention uses this quadratic response characteristic to producea linear net output force in a manner somewhat analogous to thatproduced according to the method of Murray et al.

Various preferred modes of practicing the present invention include whatare referred to herein as a “first” mode of inventive practice and a“second” mode of inventive practice. According to both the first modeand the second mode of practicing the present invention, the outputforce is linear over the magnetic flux density range for which themagnetostrictive material response is quadratic.

According to the first mode of practice of the present invention, acenter radial bias magnet ring drives flux through two magnetostrictiveelements in opposite directions while surrounding drive coils applymagnetic flux in the same direction through the two elements. The forceoutput connection is between the two elements; therefore, the net outputis the difference of the forces respectively generated in the twomagnetostrictive elements.

According to the second mode of practice of the present invention, acenter radial bias magnet ring drives flux through a singlemagnetostrictive element in opposite directions in upper and lowerhalves while surrounding drive coils apply magnetic flux in a singledirection. The force output connection is at the center of the element;therefore, the net output is the difference of the forces respectivelygenerated in the upper and lower halves of the single magnetostrictiveelement.

Exemplary practice of the present invention achieves a substantiallyuniform magnetic flux density down the length of the magnetostrictiveelements by implementing a flux transfer tube between the center radialbias magnet ring and the magnetostrictive elements. The shape of theflux transfer tube is optimized to transfer flux into themagnetostrictive elements at the same rate as it leaks out, resulting inno net loss of flux down the length of the elements.

Some embodiments of the present invention provide for implementation ofa flux transfer tube, whereas other inventive embodiments do not.Inventive use of the flux transfer tube is optional, depending on themagnetic permeability of the material used and the distance between themagnetostrictive elements and the surrounding flux return. For Galfenol,acceptable performance may be achieved without a flux transfer tube ifthe element length is less than about four times the separationdistance. Because Terfenol has a lower magnetic permeability, a fluxtransfer tube is generally necessary if the element length is more thanabout two times the separation distance.

Some embodiments of the present invention impose a mechanicalcompressive preload on the magnetostrictive elements using one or moresprings (e.g., metal spring or elastomeric spring) so as to afford amechanical compressive preload. For instance, the spring can be ahelical (e.g., cambered) metal spring or a resilient (e.g., rubber)disk. These embodiments are preferred for magnetostrictive materialsthat benefit from compressive preload, such as Terfenol. Inventiveembodiments that do not implement a spring are preferred formagnetostrictive materials that benefit from and can withstand tensilepreload, such as Galfenol.

This United States patent application is related to U.S. nonprovisionalpatent application Ser. No. 16/136,742, filing date 20 Sep. 2018, herebyincorporated herein by reference, entitled “Linear MagnetostrictiveActuator,” joint inventors John E. Miesner and George G. Zipfel, Jr.,which claims the benefit of U.S. provisional patent application No.62/564,100, filing date 27 Sep. 2017, hereby incorporated herein byreference, entitled “Linear Magnetostrictive Actuator,” joint inventorsJohn E. Miesner and George G. Zipfel.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, withreference to the accompanying drawings, wherein like numbers indicatesame or similar parts or components, and wherein:

FIG. 1 is a cross-sectional view of an exemplary embodiment of the firstmode of practice of the present invention.

FIG. 2 is an exploded view of the inventive embodiment illustrated inFIG. 1.

FIG. 3 shows an example of the calculated magnetic flux lines with nodrive current for an embodiment of the first mode of inventive practice,such as shown by way of example in FIG. 1.

FIG. 4 shows an example of the calculated magnetic flux lines at maximumdrive current for an embodiment of the first mode of inventive practice,such as shown by way of example in FIG. 1.

FIG. 5 shows an example of the calculated magnetic flux density for acontour line down the magnetostrictive elements at no drive current andat maximum drive current for an embodiment of the first mode ofinventive practice, such as shown by way of example in FIG. 1.

FIG. 6 shows an example of the calculated magnetic flux lines with nodrive current for an embodiment of the first mode of inventive practice,such as shown by way of example in FIG. 1.

FIG. 7 is a cross-sectional view of an exemplary embodiment of thesecond mode of practice of the present invention.

FIG. 8 is an exploded view of the inventive embodiment illustrated inFIG. 7.

FIG. 9 shows an example of the calculated magnetic flux lines with nodrive current for an embodiment of the second mode of inventivepractice, such as shown by way of example in FIG. 7.

FIG. 10 shows an example of the calculated magnetic flux lines atmaximum drive current for an embodiment of the second mode of inventivepractice, such as shown by way of example in FIG. 7.

FIG. 11 shows an example of the calculated magnetic flux density for acontour line down the magnetostrictive element at no drive current andat maximum drive current for an embodiment of the second mode ofinventive practice, such as shown by way of example in FIG. 7.

DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

FIG. 1 and FIG. 2 are two views exemplifying the first mode of practiceof a magnetostrictive actuator in accordance with the present invention.The inventive actuator depicted includes a magnetostrictive component312, which includes two magnetostrictive elements 101, viz., 101 a and101 b. Top magnetostrictive element 101 a and bottom magnetostrictiveelement 101 b are each in contact with center support 102 such thatchanges in the respective lengths of the magnetostrictive elements 101correspondingly move center support 102 in the axial direction a. Centersupport 102 is moved vertically, either upward or downward, as shown bybidirectional arrow a in FIG. 1. Output shaft 103 is connected to centersupport 102 and therefore moves in one of two oppositeaxial-longitudinal directions in accordance with the axial-longitudinalmovement of center support 102.

Output shaft 103 has an inner shaft end (lower shaft end 1032 as shownin FIG. 1) and an outer shaft end (upper shaft end 1031 as shown in FIG.1). Output shaft 103 is attached to center support 102 at lower shaftend 1032. Output shaft 103 moves with center support 102 in axialdirection a. Support bearing 104 provides radial support to output shaft103 while allowing output shaft 103 to move in axial direction a. Acurrent-carrying coil component 352 includes two current-carrying coils105, viz., coils 105 a and 105 b, and surrounds magnetostrictiveelements 101. That is, top coil 105 a and bottom coil 105 b surroundmagnetostrictive elements 101 a and 101 b, respectively, wherein coils105 a and 105 b produce a fluctuating magnetic field in magnetostrictiveelements 101 a and 101 b that is proportional to the electrical current.According to exemplary inventive practice, top coil 105 a and bottomcoil 105 b are congruous, such as shown in FIG. 1. However, depending onthe inventive embodiment, coils 105 a and 105 b may have the same shapeor may differ in shape, such as having equal or unequal axial lengths.

The magnetostrictive elements 101 a and 101 b change length inaccordance with their magnetostrictive characteristics, moving centersupport 102 and output shaft 103 to produce useful work in response tothe coil current. Optimum actuator output may be inventively obtainedfor magnetostrictive materials such as Terfenol when the elements 101 aand 101 b have a mechanical compressive preload and a magnetic bias. Forinstance, as shown in FIG. 1, the mechanical compressive preload isprovided by spring 106, which presses down on top support 107 which isin contact with top magnetostrictive element 101 a. Spring 106 pressesup against cylinder closure 108, which also supports bearing 104.

Magnetic bias is provided by radially polarized magnet ring 109, whichis in contact with flux transfer tube 110. Flux transfer tube 110surrounds center support 102 and magnetostrictive elements 101 a and 101b, and transfers magnetic flux to magnetostrictive elements 101 a and101 b from magnet ring 109. Flux transfer tube 110 is designed toprovide a substantially uniform magnetic bias down the length ofmagnetostrictive elements 101 a and 101 b. As shown in FIG. 1 and FIG.2, flux transfer tube 110 is cylindrical having a thick wall in theaxial middle, where the flux transfer tube 110 attaches to the biasmagnet ring 109. The wall thickness of flux transfer tube 110 decreasestoward its two axial ends to form a tapered outer surface.

It should be noted that flux from radially polarized magnet ring 109will flow oppositely in top magnetostrictive element 101 a and bottommagnetostrictive element 101 b. For example, if the flux direction is upin top magnetostrictive element 101 a, then it will be down in bottommagnetostrictive element 101 b. It should be further noted that the fluxdirection from current-carrying coils 105 a and 105 b is in the samedirection in both magnetostrictive elements 101 a and 101 b.

Therefore, the total flux at any particular current level will be higherthan the bias flux in one magnetostrictive element 101, and will belower than the bias flux in the other magnetostrictive element 101.Consequently, as electrical current increases, one of themagnetostrictive elements 101 will be elongating and the other will beshortening, thereby moving center support 102 either up or down in axialdirection a. When the current reverses, center support 102 will reversedirection along axial direction a. The flux path for both radiallypolarized magnet ring 109 and for drive coils 105 a and 105 b iscompleted by top support 107, top flux return 111, cylindrical fluxreturn 112, and bottom flux return 113, which are preferably made of amaterial such as silicon steel to provide high permeability and lowhysteresis, and which may for example be composed of flat or spiralwound laminations.

It should be noted that magnetostrictive elements 101 a and 101 b areeach cylindrical in nature, with an axial hole down the length, and aresituated coaxially along their respective lengths. As shown in FIG. 1, astraight output shaft 103 passes through the axial-longitudinal hole oftop magnetostrictive element 101 a. According to exemplary inventiveembodiments, magnetostrictive elements 101 a and 101 b are geometricallycongruous (i.e., have the same axial length, diameter, and wallthickness), such as shown in FIG. 1, and are each homogenous. However,depending on the inventive embodiment, magnetostrictive elements 101 aand 101 b may have the same shape or may differ in shape, such as havingequal or unequal axial lengths.

Furthermore, in inventive practice, neither magnetostrictive element 101a nor magnetostrictive element 101 b need be homogeneous, and eitherelement or both elements may be composed of sub-elements. For example,either or both of magnetostrictive elements 101 a and 101 b may becomposed of radial, transverse, or axial laminates, or may be composedof an array of rods or bars of arbitrary cross-section. An essentialconsideration in inventive first-mode practice is that magnetostrictiveelements 101 a and 101 b are disposed uniformly around output shaft 103and provide uniform ends upon which bottom flux return 113, centersupport 102, and top support 107 can bear.

Radially polarized magnet ring 109 is preferably composed, for example,of a series of high strength magnet segments, and is in contact with andbonded to flux transfer tube 110. Flux transfer tube 110 is preferablymade of a material such as silicon steel to provide high permeabilityand low hysteresis. Flux transfer tube 110 preferably includes aninterruption, such as vertical slit 115, to prevent eddy currents fromcirculating circumferentially.

FIG. 3 shows an axisymmetric magnetic model exemplifying the first modeof practice of the present invention under condition of no current flowthrough the drive coils 105. The magnetostrictive material is Terfenolfor this example. The calculated magnetic flux paths are illustrated bylines F. With no drive coil current flow, all flux lines F flow frombias magnet 109 and form closed upper and lower loops back to biasmagnet 109. It can be seen in FIG. 3 that top magnetostrictive element101 a and bottom magnetostrictive element 101 b have respective biasfluxes in opposite directions from each other.

FIG. 4 shows the same axisymmetric magnetic model as FIG. 3, but undercondition of maximum rated current flow through drive coils 105 a and105 b. In the example illustrated in FIG. 4, the magnetic flux from thedrive coils and from bias magnet 109 reinforces in the bottommagnetostrictive element 101 b and cancels in the top magnetostrictiveelement 101 a. Thus the bottom magnetostrictive element 101 b elongatesand the top magnetostrictive element 101 a shortens with respect to theno-drive-current condition moving center support 102 in the upwarddirection. If the direction of current flow is reversed, then themagnetic flux from the drive coils and from bias magnet 109 will cancelin the bottom magnetostrictive element 101 a and reinforce in the topmagnetostrictive element 101 b, thereby moving center support 102 in thedownward direction.

FIG. 5 is a plot of the calculated flux density from the examples ofFIG. 3 and FIG. 4, for an axial contour at a radius halfway between thecenter hole and the outer surface for magnetostrictive elements 101 aand 101 b. It can be seen in FIG. 5 that under condition of no currentflow through the drive coils 105, the bias flux is nearly constant at0.6 Tesla down the length of the magnetostrictive elements 101. Undercondition of maximum rated current flow, the magnetic flux density inmagnetostrictive element 101 a is nearly zero, while the magnetic fluxdensity in magnetostrictive element 101 b is about 0.9 Tesla. If thedirection of current flow were reversed, these flux density values wouldalso reverse.

FIG. 6 shows the same exemplary axisymmetric magnetic model as FIG. 3,but with the flux transfer tube 110 removed. The inventive embodimentshown in FIG. 3 has bias magnet ring 109 and flux transfer tube 110. Theinventive embodiment shown in FIG. 6 has bias magnet ring 109 a and doesnot have a flux transfer tube such as flux transfer tube 110. Biasmagnet ring 109 a (shown in FIG. 6) has a smaller inner diameter thanbias magnet ring 109 (shown in FIG. 3), to compensate for the removal.It can be seen in FIG. 6 that without flux transfer tube 110, fluxleakage from magnetostrictive elements 101 a and 101 b causes the numberof flux lines to reduce with distance from bias magnet 111, whichindicates that the magnetic flux density is decreasing. With the fluxtransfer tube 110 in place as in FIG. 3, the number of flux lines isconstant in magnetostrictive elements 101 a and 101 b, indicating asubstantially uniform magnetic flux density.

In practicing many embodiments of the first mode of the presentinvention, the optimum shape of flux transfer tube 110 may be calculatedby a person having ordinary skill in the art who reads the instantdisclosure. The ordinarily skilled artisan may perform this calculationusing a magnetic model and adjusting geometric parameters until the fluxis at the desired level and within acceptable bounds of uniformity. Theoptimum shape of flux transfer tube 110 depends upon the magneticpermeability of the magnetostrictive material, and is a compromisebecause the permeability varies with magnetic flux level and stress. Fora low permeability material, such as Terfenol, the flux transfer tube110 is cylindrical with a thick wall in the axial middle, where fluxtransfer tube 110 attaches to the bias magnet ring 109, and with thewall thickness decreasing toward the two axial ends of flux transfertube 110 to form a tapered outer surface. The taper angle corresponds tothe flux density reduction rate as the flux leaks out down the length ofthe tube 110 as shown in FIG. 3. The optimum taper angle is realizedwhen the flux lines in magnetostrictive elements 101 a and 101 b areconstant and vertical as shown in FIG. 3.

FIG. 7 and FIG. 8 are two views of the second mode of practice of thepresent invention. The inventive actuator depicted in FIGS. 7 and 8includes a magnetostrictive component 312, which includes onemagnetostrictive element, viz., magnetostrictive element 201. Accordingto this inventive example, magnetostrictive component 312 includes asingle magnetostrictive element 201, which is characterized by a topmagnetostrictive element half 201T and a bottom magnetostrictive elementhalf 201B. Magnetostrictive element 201 is attached to center support202 such that change from center position of magnetostrictive element201 correspondingly moves center support 202 in the axial direction a.Center support 102 is moved vertically, either upward or downward, asshown by bidirectional arrow a in FIG. 7. Output shaft 203 is connectedto center support 202 and therefore moves in one of two oppositeaxial-longitudinal directions in accordance with the axial-longitudinalmovement of center support 202.

Output shaft 203 has an inner shaft end (lower shaft end 2032 as shownin FIG. 1) and an outer shaft end (upper shaft end 2031 as shown in FIG.1). Output shaft 203 is attached at its lower shaft end 2032 to centersupport 202. Output shaft 203 moves with center support 202 in the axialdirection a. Support bearing 204 provides radial support to output shaft203 while allowing it to move in axial direction a. Two current-carryingcoils 205—viz., top coil 205 a and bottom coil 205 b—surroundmagnetostrictive element 201, producing a fluctuating magnetic field inthe element proportional to the current. According to exemplaryinventive practice, top coil 205 a and bottom coil 205 b are congruous.The top and bottom halves 201T and 201B of magnetostrictive element 201each change length in accordance with its magnetostrictivecharacteristics, thereby moving center support 202 and output shaft 203to produce useful work in response to the coil current.

For a magnetostrictive material such as Galfenol, optimum actuatoroutput is obtained whenever the magnetostrictive element 201 has amechanical tensile preload and a magnetic bias. The mechanical tensilepreload is provided by spring 206, which presses upon top support 207,which is attached to magnetostrictive element 201. Bottom support 214 isalso attached to magnetostrictive element 201 and transfers tensileforce to bottom flux return 213. Spring 206 presses down against topflux return 211. Cylinder closure 208 supports bearing 204.

Magnetic bias is provided by radially polarized magnet ring 209, whichis in contact with flux transfer tube 210. Flux transfer tube 210surrounds center support 202 and magnetostrictive element 201 andtransfers magnetic flux to it from magnet ring 209. Flux transfer tube210 is designed to provide a substantially uniform magnetic bias downthe length of magnetostrictive element 201. It should be noted that fluxfrom radially polarized magnet ring 209 will flow oppositely in the tophalf 201T of magnetostrictive element 201 and the bottom half 201B. Forexample, if the flux direction is upward in the top half 201T ofmagnetostrictive element 201, then the flux direction will be down inthe bottom half 201B of magnetostrictive element 201. It should befurther noted that the flux direction from current carrying coils 205 aand 205 b is in the same direction through the length ofmagnetostrictive element 201.

Therefore, the total flux at any particular current level will be higherthan the bias flux in one half of magnetostrictive element 201, andlower in the other half of magnetostrictive element 201. Consequently,as current increases, one of the two halves of magnetostrictive element201 will be elongating and the other will be shortening, thereby movingcenter support 202 in the axial direction. For instance, element tophalf 201T lengthens and element bottom half 201B shortens; or, elementbottom half 201B lengthens and element top half 201T shortens. When thecurrent reverses, center support 202 will also reverse direction. Theflux path for both radially polarized magnet ring 209 and for drivecoils 205 a and 205 b is completed by top support 207, top flux return211, cylindrical flux return 212, and bottom flux return 213, which arepreferably made of a material such as silicon steel to provide highpermeability and low hysteresis, and which may for example be composedof flat or spiral wound laminations.

It should be noted that magnetostrictive element 201 is cylindrical innature, with an axial hole down the length, but need not be homogeneousand may be composed of sub-elements. For example, magnetostrictiveelement 201 may be composed of radial, transverse, or axial laminates ormay be composed of an array of rods or bars of arbitrary cross section.An essential consideration is that magnetostrictive element 201 isdisposed uniformly around output shaft 203 and allows attachment tobottom flux return 213, center support 202, and top support 207 bybonding, welding, or mechanical means. For Galfenol, the preferredarrangement for magnetostrictive element 201 is a circumferential arrayof flat bars as shown in FIG. 8. Magnetostrictive elements 101 a and 101b (shown FIG. 1 and FIG. 2) are cylindrical, whereas magnetostrictiveelement 201 (shown FIG. 1 and FIG. 2) is hexagonally prismatic. As usedherein, the term “at least substantially cylindrical” refers to anaxially symmetrical shape such as a cylinder or a prism having at leastsix sides.

Radially polarized magnet ring 209 is preferably composed of a series ofhigh strength magnet segments and is in contact with and bonded to fluxtransfer tube 210. Flux transfer tube 210 is preferably made of amaterial such as silicon steel to provide high permeability and lowhysteresis. Flux transfer tube 210 preferably includes an interruption,such as vertical slit 215, to prevent eddy currents from circulatingcircumferentially.

FIG. 9 shows an axisymmetric magnetic model exemplifying the second modeof practice of the present invention under condition of no current flowthrough the drive coils. The magnetostrictive material is Galfenol forthis example. The calculated magnetic flux paths are illustrated bylines F. With no drive coil current flow, all flux lines F flow frombias magnet 209 and form closed upper and lower loops back to biasmagnet 209. It can be seen in FIG. 9 that top half 201T and bottom half201B of magnetostrictive element 201 have respective bias fluxes inopposite directions from each other.

FIG. 10 shows the same axisymmetric magnetic model as FIG. 9 but undercondition of maximum rated current flow through drive coils 205 a and205 b. In this example, the respective magnetic fluxes from the drivecoils 205 a and 205 b from bias magnet 209 reinforce in the bottom halfof magnetostrictive element 201 and cancel in the top half. Thus thebottom half 201B of magnetostrictive element 201 elongates and the tophalf 201T shortens with respect to the no-drive-current condition movingcenter support 202 in the upward direction. If the direction of currentflow is reversed then the magnetic flux from the drive coils 205 a and205 b and from bias magnet 209 will cancel in the bottom half 201B ofmagnetostrictive element 201 and will reinforce in the top half 201T,thereby moving center support 202 in the downward direction.

FIG. 11 is a plot of the calculated flux density from the FIG. 9 andFIG. 10 examples for an axial contour at a radius half way between thecenter hole and the outer surface for magnetostrictive element 201. Itcan be seen that under condition of no current flow through the drivecoils 205 a and 205 b, the bias flux is substantially constant at 0.55Tesla down the length of the element 201. Under condition of maximumrated current flow the magnetic flux density in the bottom half 201B ofmagnetostrictive element 101 a is nearly zero, while the magnetic fluxdensity in the top half 201T is about 1.05 Tesla. If the direction ofcurrent flow were reversed then these flux density values would alsoreverse.

In practicing many embodiments of the second mode of the presentinvention, the optimum shape of flux transfer tube 210 may be calculatedby a person having ordinary skill in the art who reads the instantdisclosure. The ordinarily skilled artisan may perform this calculationusing a magnetic model and adjusting geometric parameters until the fluxis at the desired level and within acceptable bounds of uniformity. Theoptimum shape of flux transfer tube 210 depends upon the magneticpermeability of the magnetostrictive material, and is a compromisebecause the permeability varies with magnetic flux level and stress. Fora higher permeability material, such as Galfenol, the optimum tube wallthickness of flux transfer tube 210 is nearly uniform down the lengthbut flares outward with the diameter increasing toward the ends, such asshown in FIG. 7 and FIG. 8. The optimum flare angle is realized when theflux lines in magnetostrictive elements 201 are constant and vertical,such as shown in FIG. 9.

The present invention, which is disclosed herein, is not to be limitedby the embodiments described or illustrated herein, which are given byway of example and not of limitation. Other embodiments of the presentinvention will be apparent to those skilled in the art from aconsideration of the instant disclosure, or from practice of the presentinvention. Various omissions, modifications, and changes to theprinciples disclosed herein may be made by one skilled in the artwithout departing from the true scope and spirit of the presentinvention, which is indicated by the following claims.

What is claimed is:
 1. A magnetostrictive actuator comprising a coilcomponent, a magnetostrictive component situated inside said coilcomponent, an output shaft situated inside said magnetostrictivecomponent, and a support member situated inside said coil component andattached to said magnetostrictive component and said output shaft,wherein: said coil component, said magnetostrictive component, saidoutput shaft, and said support member are each at least substantiallycylindrical and are characterized by the same geometric longitudinalaxis; said coil component is capable of carrying current and ofproducing a fluctuating magnetic field in said magnetostrictivecomponent that is proportional to said current carried by said coilcomponent; said magnetostrictive component magnetostrictively changes inshape in accordance with said fluctuating magnetic field; said supportmember and hence said output shaft move in an axial direction inaccordance with said magnetostrictive changing in shape of saidmagnetostrictive component; wherein said magnetostrictive componentincludes two magnetostrictive elements each characterized by saidgeometric longitudinal axis; wherein said support member is situatedbetween said two magnetostrictive elements.
 2. The magnetostrictiveactuator of claim 1, wherein said magnetostrictive change in shape ofsaid magnetostrictive component in accordance with said fluctuatingmagnetic field includes: an increase in axial length of a first saidmagnetostrictive element; and a decrease in axial length of a secondsaid magnetostrictive element.
 3. A magnetostriction-based actuationdevice characterized by a geometric longitudinal axis, themagnetostriction-based actuation device comprising: a shaft; a firstmagnetostrictive element, said first magnetostrictive element being atleast substantially cylindrical and encompassing said shaft; a secondmagnetostrictive element, said second magnetostrictive cylinder being atleast substantially cylindrical and encompassing said shaft, said secondmagnetostrictive cylinder axially aligned with said firstmagnetostrictive cylinder; a first coil, said first coil encompassingsaid first magnetostrictive element; a second coil, said second coilencompassing said second magnetostrictive element, said second coilaxially aligned with said first coil; a support member, said supportmember placed axially between and coupling said first magnetostrictiveelement and said second magnetostrictive element, said support memberattached to said shaft at an axial end of said shaft; a magnet ring,said magnet ring placed axially between and coupling said first coil andsaid second coil; a flux transfer tube, said flux transfer tubeencompassing said first magnetostrictive element, said secondmagnetostrictive element, and said support member, said flux transfertube encompassed by said first coil, said second coil and said magnetring, said flux transfer tube contacting said magnet ring; wherein saidshaft, said first magnetostrictive element, said second magnetostrictiveelement, said first coil, said second coil, said support member, saidmagnet ring, and said flux transfer tube are coaxially arranged withrespect to said axis; wherein in accordance with a fluctuating magneticfield produced by current carried by said first coil and said secondcoil, one of said first magnetostrictive element and said secondmagnetostrictive element increases in axial length, and the other ofsaid first magnetostrictive element said second magnetostrictive elementdecreases in axial length; wherein in accordance with said increasingand decreasing in axial length of said first magnetostrictive elementand said second magnetostrictive cylinder, said shaft moves in either oftwo axial directions; wherein the combination including said magneticring and said flux transfer tube produces an at least substantiallyuniform magnetic bias along said axis.
 4. The magnetostriction-basedactuation device of claim 3, further comprising an exterior case and aspring, said spring being associated with said exterior case andproviding a compressive preload in an axial direction with respect tosaid shaft.
 5. The magnetostriction-based actuation device of claim 3,wherein said flux transfer tube is cylindrical having a tube wall, atube axial middle and two tube axial ends, said tube wall being thick atsaid tube axial middle and attaching to said bias magnet ring at saidtube axial middle, the thickness of said tube wall decreasing towardsaid two tube axial ends to form a tapered outer surface of said tubewall.
 6. The magnetostriction-based actuation device of claim 5, furthercomprising an exterior case and a spring, said spring being associatedwith said exterior case and providing a compressive preload in an axialdirection with respect to said shaft.